Why [calendar dateFromComponents:] always return hour 16 ? - ios

Question

When I pass the [NSDate date] to NSDateComponents , then pass back after the weekday modified, I always got the hour 16:00:00 . Why ?
Code belows:
{
NSCalendar *calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDate *now = [NSDate date];
NSLog(#"now = %#",now);
NSDateComponents *firstDayOfWeek = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitWeekday fromDate:now];
firstDayOfWeek.weekday = 1; // Monday as first day of week
NSDate *firstDayOfWeekDate = [calendar dateFromComponents:firstDayOfWeek];
NSLog(#"first day = %#", firstDayOfWeekDate);
}
{
NSCalendar *calendar = [[NSCalendar alloc]initWithCalendarIdentifier:NSCalendarIdentifierGregorian];
NSDate *now = [NSDate date];
NSLog(#"now = %#",now);
NSDateComponents *firstDayOfWeek = [calendar components:NSCalendarUnitYear | NSCalendarUnitMonth | NSCalendarUnitDay | NSCalendarUnitWeekday | NSCalendarUnitHour | NSCalendarUnitMinute | NSCalendarUnitSecond fromDate:now];
firstDayOfWeek.weekday = 1; // Monday as first day of week
firstDayOfWeek.hour = 1;
firstDayOfWeek.minute = 1;
firstDayOfWeek.second = 1;
NSDate *firstDayOfWeekDate = [calendar dateFromComponents:firstDayOfWeek];
NSLog(#"first day = %#", firstDayOfWeekDate);
}
And the output is :
now = 2016-01-18 03:14:08 +0000
first day = 2016-01-17 16:00:00 +0000
now = 2016-01-18 03:14:08 +0000
first day = 2016-01-17 17:01:01 +0000
Why the hour begins with 16 ?

Answer 1

NSDateComponents isn't making it 16:00:00. It's making it midnight in your current timezone. But the NSLog is showing you the equivalent GMT time. That's what the +0000 means: e.g. GMT +00:00.
Bottom line, if you want to see the resulting NSDate object in your local time zone, you should use NSDateFormatter.

Answer 2

This is due to timezone.
Try to add NSDateFormatter before you output the date with NSLog, e.g.
NSDateFormatter *dateFormatter = [[NSDateFormatter alloc] init];
[dateFormatter setLocale:[NSLocale currentLocale]];
[dateFormatter setDateStyle:NSDateFormatterShortStyle];
[dateFormatter setTimeStyle:NSDateFormatterLongStyle];
NSString *dateDisplay = [dateFormatter stringFromDate:firstDayOfWeekDate];
NSLog(#"first day = %#",dateDisplay);
You can refer this article for more examples.